This invention relates to techniques for electromagnetically and rotationally-vibrating a rotary body supported by bearings, and more particularly relates to an electromagnetic rotary vibrator, for a rotary body, which is adapted to apply a rotational exciting force to the rotary body by using a bipolarly magnetized ring-shaped magnet and a ring-shaped stator which are adapted to generate a rotating magnetic field by controlling an electric current that is supplied to a coil. This invention also includes a damper for a rotary body which uses the above-described vibrator.
A rotary machine has a critical speed at which a vibration amplitude increases suddenly when a r evolution velocity of the rotary body increases to coincide with a natural frequency. A rotary body used to pass a critical speed is subjected to a test for the purpose of minimizing a vibration amplitude generated by the influence of such a use, thereby evaluating the balance of the vibration of the rotary body and carrying out a suitable balancing operation. Rotary machines, such as first-order passing type to N-th order passing type rotary machines, are especially adapted to pass critical speeds to reach a rated revolution velocity and are thus subjected to tests so as to pass the critical speeds, in the same manner as the above-mentioned rotary body, and a suitable balancing operation is carried out because otherwise, a rated revolution velocity cannot be attained. A balancing, operation for a rotary body is necessarily carried out in this manner.
According to the conventional balancing techniques, a rotary body is installed in an operable rotary machine, and unbalanced vibration is measured with the rotary machine in operation. On the basis of the results of measurement, either a balance weight is attached to the rotary body, or conversely, the rotary body is shaved, whereby the vibration of the rotary body is suppressed. In an actual rotation test, the determination of the vibrational amplitude and the calculation and correction of balance have to be performed at each and every critical speed. Namely, very complicated operations requiring extensive labor and time have to be carried out.
In recent years, a displacement feedback control method using a controlling type magnetic bearing has been studied as one method of solving these problems. A controlling type magnetic bearing is originally adapted to apply a force to a rotary shaft so that the rotary shaft is constantly maintained in a neutral position in the same bearing. In a damping operation using this controlling type magnetic bearing, an exciting force offsetting an unbalanced vibration of a rotary body, and synchronous with the rotation of the body, is applied in a superposed manner to the controlling type magnetic bearing so as to suppress the vibration and allow the body to pass critical speeds so that balance evaluation can be carried out. Such a method is disclosed in, for example, Japanese Patent Laid-Open Application No. 4-351348/1992.
A controlling type magnetic bearing is basically adapted to attract a magnetic substance of a rotary shaft to electromagnetic actuators and retain the rotary shaft in a neutral position in the bearing. The electromagnetic actuators, the attractive force of two poles which act on one position, are used by arranging them in four directions of an XY plane (a plane perpendicular to a direction Z in which the rotary shaft extends). In order to generate a rotating magnetic field for applying a exciting force to a rotary body, rotational synchronous single-phase electric power of sine and cosine waves is generated by using a personal computer-controlled DSP (digital signal processor) or a tracking filter circuit, and the generated rotational synchronous single-phase electric power is superposed to the neutral position, which retains electric power, of the electromagnetic actuators arranged in the XY plane.
However, since the controlling type magnetic bearings are originally adapted to apply a force to, and control, a rotary shaft so that the rotary shaft is retained constantly in a neutral position in the bearings as mentioned above, an exciting force, offsetting unbalanced vibration and synchronous with the rotation of the rotary body, constitutes a disturbance to the magnetic bearings and causes the action of the bearings to become unstable. Namely, an exciting force due to the effect of the attractive rotating magnetic field of the controlling type magnetic bearings is generated for obtaining a spring effect by displacement feedback and a damping effect by speed feedback, and the original function of the controlling type magnetic bearings works so as to offset this disturbance (exciting force). Therefore, a very difficult control operation is required to correct the unstable bearings.
The functioning of the electromagnetic actuators which attract the magnetic substance of the rotary shaft and which are arranged in four directions of XY plane, work only solely at an instant, and they do not work with full power, such that an exciting force is small. This causes the dimensions of the electromagnetic actuators to increase. Regarding a control operation, problems such as a feedback delay occur. Although the electromagnetic actuators basically work so as to attract the rotary body, an unnecessary rotational force is also produced, and this causes whirling (self-excited vibration of the rotary body). Moreover, in view of the DSP response characteristics, the attainment of a high revolution velocity and the passage of a high-order critical speed are difficult to achieve in the foregoing.